As far as I can understand, the primary (but not only) reason behind the 500ml minimum requirement is that early kettle designs included a heating element that was above the base of the kettle (something like this: https://media.sciencephoto.com/image/h1300610/800wm/H1300610...) and for safe and effective operation of the kettle, the heating element had to be covered by water.

I do think there is another reason for the minimum quantity requirement (increasing the likelihood that the generated steam will trigger the temperature sensor) but this other reason is far less important.

In other words, almost all modern kettles don't actually need a minimum of 500ml of water - maybe just 100ml or so to ensure water does turn into steam and triggers the temperature sensor.

Kettle producers have just never really updated the "minimum amount" line. Though I have occasionally started to see kettles without this line.